Heterogeneously catalyzed chemical reduction of carbon dioxide

ABSTRACT

The presently disclosed and/or claimed inventive concept(s) relates generally to the reduction of carbon dioxide by heterogeneous catalysis. More particularly, but not by way of limitation, the presently disclosed and/or claimed inventive concept(s) relates to the reduction of carbon dioxide by heterogeneous catalysis with a heterogeneous hydrogenation catalyst comprising structurally frustrated Lewis pairs, wherein, for example but not by way of limitation, formic acid is produced and hydrocarbons are indirectly produced. In one non-limiting embodiment, the heterogeneous catalyst comprises hexagonal boron nitride (h-BN) having structurally frustrated Lewis pairs therein.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCESTATEMENT

The present application claims benefit under 35 U.S.C. 119(e) of U.S.Provisional Patent Application Ser. No. 61/734,699, filed Dec. 7, 2012,and 61/860,067, filed Jul. 30, 2013, the entire contents of each ofwhich is hereby expressly incorporated herein by reference.

BACKGROUND

1. Field of the Inventive Concept(S)

The presently disclosed and/or claimed inventive concept(s) relatesgenerally to the reduction of carbon dioxide by heterogeneous catalysis.More particularly, but not by way of limitation, the presently disclosedand/or claimed inventive concept(s) relates to the reduction of carbondioxide by heterogeneous catalysis with a heterogeneous hydrogenationcatalyst comprising structurally frustrated Lewis pairs, wherein, forexample but not by way of limitation, formic acid is produced andhydrocarbons are indirectly produced. In one non-limiting embodiment,the heterogeneous catalyst comprises hexagonal boron nitride (h-BN)having structurally frustrated Lewis pairs therein.

2. Background of the Inventive Concept(S)

Carbon dioxide (CO₂), a well-known greenhouse gas, is the main productemitted by the combustion of hydrocarbons for the generation of powerfor many uses including electricity and transportation. In 2012, CO₂production was at a record high of 31.6 gigatons, a number that willcontinue to escalate as consumer demands increase. As such, it iscritically important to the environment to reduce the emissions of CO₂.

Existing approaches to reducing carbon dioxide emissions includesequestration, electrochemical reduction, and homogeneous reduction.However, each of these processes has specific disadvantages. Forexample, sequestration is limited to the space available to store CO₂,electrochemical reduction is energy intensive, and homogeneous reductionrequires the utilization of catalysts that are sensitive to both air andmoisture. Additionally, existing approaches generally use catalystscomprising precious metals such as palladium, platinum, nickel, andrhodium, which increases the cost of CO₂ reduction and depletes thesupply of these precious metals. These and other factors make thepresently disclosed and/or claimed process of using a heterogeneoushydrogenation catalyst, to not only reduce CO₂ emissions but alsoproduce a commercially valuable product, an attractive alternative toexisting approaches.

Frustrated Lewis Pair (FLP) catalysts are potentially useful as one typeof heterogeneous hydrogenation catalyst. In 2007, Stephan and his teamdeveloped a chemical system capable of releasing and absorbing molecularhydrogen using frustrated Lewis pairs. (See Frustrated Pairs, inCatalysis without Precious Metals, M. R. Bullock, Editor. 2010,Wiley-VCH Verlag GmbH & Co. KGaA. P.I-XVIII, hereby incorporated in itsentirety). Stephan determined that when a sterically encumbered Lewisacid approaches a bulky Lewis base, adduct formation is hindered andgives rise to electronic “frustration”. Such frustration effectivelymimics the donor-acceptor properties of transition metals. Stephan andhis team demonstrated that upon exposure to 1 atm H₂ at 25° C., asolution of red phosphino-borane [(C₆H₂Me₃-2,4,6)₂P(C₆F₄)BF(C₆F₅)₂]transformed to the colorless zwitterionic salt[(C₆H₂Me₃-2,4,6)₂PH(C₆F₄)BH(C₆F₅)₂]. Upon thermolysis at 150° C., thesalt lost H₂ and converted back to the original phosphine-boranesubstrate. Such phosphonium borates (as well as similar compounds) havebeen shown to successfully catalyze the hydrogenation of select imines,enamines, aldehydes, and olefins.

However, to date, only homogeneous FLP catalytic systems have beenstudied, i.e., FLP catalysts and the resulting reactions have previouslyinvolved the FLP catalyst being in the same phase as the reactants. Forexample, the homogeneous FLP catalyst is typically co-dissolved in asolvent with the reactants. Heterogeneous catalysis, on the other hand,is performed with the catalyst in a different phase from that of thereactants. One example of heterogeneous catalysis is the petrochemicalalkylation process where the liquid reactants are immiscible with asolution containing the catalyst. Heterogeneous catalysis offers theadvantage that products may be readily separated from the catalyst.Typically, heterogeneous catalysts are more stable and degrade muchslower than homogeneous catalysts.

As detailed in the presently disclosed and/or claimed invention,heterogeneous catalysts, e.g., heterogeneous FLP catalysts, can be usedto hydrogenate the carbonyl bond of carbon dioxide to produce formicacid, which as of 2013 has a commodity price of 700-1,000 USD/metric tonand can be used as a fuel or in fuel cells. Additionally, formic acidcan be thermally decomposed to produce carbon monoxide and water,wherein the carbon monoxide can be further converted to a hydrocarbonfuel using Fischer-Tropsch chemistry. Given that on average a typicalpower plant emits 10,000 tons of CO₂ per day, the ability to efficientlyturn CO₂ into a commodity while reducing the emissions of CO₂ is avaluable alternative to processes currently available for industrial andcommercial use.

In view of the foregoing, there is a need for a heterogeneous catalystcapable of chemically reducing CO₂ in order to efficiently decrease theproduction of CO₂ emissions. In particular, a heterogeneoushydrogenation catalyst having an FLP-type electronic structure would bea valuable addition to catalysts currently available for industrial andcommercial use. It is to such a heterogeneous hydrogenation FLP catalystand its method of use that the presently disclosed and/or claimedinventive concept(s) is directed.

SUMMARY OF THE INVENTIVE CONCEPTS

The presently disclosed and/or claimed inventive concept(s) relatesgenerally to the reduction of carbon dioxide by heterogeneous catalysiswith a heterogeneous hydrogenation catalyst comprising structurallyfrustrated Lewis pairs. At least one defect frustrates at least one pairof Lewis acid and Lewis base sites such that the frustrated pair ofLewis acid and Lewis base sites are catalytically active and promotehydrogenation. In one non-limiting embodiment, the heterogeneouscatalyst comprises hexagonal boron nitride (h-BN) having structurallyfrustrated Lewis pairs therein. Methods of preparing and using theheterogeneous hydrogenation catalyst for chemically reducing CO₂ toproduce, for example but without limitation, formic acid and hydrocarbonbyproducts, are also taught and disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the presently disclosed and/or claimed inventiveconcept(s) may be better understood when consideration is given to thefollowing detailed description thereof. Such description makes referenceto the annexed pictorial illustrations, schematics, graphs, anddrawings. The figures are not necessarily to scale and certain featuresand certain views of the figures may be shown exaggerated, to scale, orin schematic in the interest of clarity and conciseness.

FIG. 1 is a SEM image of defect laden h-BN.

FIG. 2 is an image of a solid, porous structure of defect laden boronnitride dispersed in polydimethylsilazane obtained after heating underdynamic vacuum.

FIG. 3 is a graphical representation illustrating discrete elementmodels that show that compressive forces achievable in our reactor aredependent on the milling intensity. The small cylinder utilized lessmedia and had a smaller volume. This produced lower compressive forcesand a lower hydrogenation yield (5% hydrogenation of chalcone after 24hours). Higher forces are obtained using more media and a larger volume(standard shape). Improved hydrogenation was observed (100%hydrogenation of chalcone after 24 hours).

FIG. 4 is an illustration of a mechanical reactor for the reduction ofCO₂ to formic acid.

FIG. 5 is a graphical representation of the rapid pressure loss (dashedand dotted line) measured at 170° C. resulting from the production offormic acid and its subsequent trapping as a liquid in an activatedcharcoal coal trap.

FIG. 6 is a graphical representation showing the formation of formicacid from the reduction of carbon dioxide.

FIG. 7 is a graphical representation showing the incorporation of carbononto defect laden h-BN catalyst after using the catalyst to reduce CO₂into formic acid.

FIG. 8 (left) is an image of a pebble mill reaction vessel constructedof alumina and filled with ZrO₂ milling media. The sloped sidesfacilitate circulation of the catalyst. (Right) A simulation of thereactor shows the evolution of forces that occur. At 66 rpm, forces upto 25 N are realized.

FIG. 9 is a graphical representation of the thermogravimetric analysis,in air, of the catalyst before (dashed line) and after (solid line) thereduction of carbon dioxide. As illustrated, significant amounts ofcarbon compounds are immobilized on the catalyst.

FIG. 10 is an image illustrating that carbon is incorporated onto thedefect laden h-BN catalyst after the reduction of carbon dioxide, whichresults in the catalyst turning a tan or brown shade (left side ofimage).

DETAILED DESCRIPTION

Before explaining at least one embodiment of the presently disclosedand/or claimed inventive concept(s) herein in detail, it is to beunderstood that the presently disclosed and/or claimed inventiveconcept(s) is not limited in its application to the details ofconstruction, experiments, exemplary data, and/or the arrangement of thecomponents set forth in the following description, or illustrated in thedrawings. The presently disclosed and/or claimed inventive concept(s) iscapable of other embodiments or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for purpose of description only andshould not be regarded as limiting in any way.

In the following detailed description of embodiments of the presentlydisclosed and/or claimed inventive concept(s), numerous specific detailsare set forth in order to provide a more thorough understanding of theinventive concept(s). However, it will be apparent to one of ordinaryskill in the art that the inventive concept(s) within the disclosureand/or appended claims may be practiced without these specific details.In other instances, well-known features have not been described indetail to avoid unnecessarily complicating the instant disclosure.Unless otherwise defined herein, technical terms used in connection withthe presently disclosed and/or claimed inventive concept(s) shall havethe meanings that are commonly understood by those of ordinary skill inthe art. Further, unless otherwise required by context, singular termsshall include pluralities and plural terms shall include the singular.

All patents, published patent applications, and non-patent publicationsmentioned in the specification are indicative of the level of skill ofthose skilled in the art to which the presently disclosed and/or claimedinventive concept(s) pertains. All patents, published patentapplications, and non-patent publications referenced in any portion ofthis application are herein expressly incorporated by reference in theirentirety to the same extent as if each individual patent or publicationwas specifically and individually indicated to be incorporated byreference.

All of the articles and/or methods disclosed herein can be made andexecuted without undue experimentation in light of the presentdisclosure. While the articles and methods of the presently disclosedand/or claimed inventive concept(s) have been described in terms ofpreferred embodiments, it will be apparent to those skilled in the artthat variations may be applied to the articles and/or methods and in thesteps or in the sequence of steps of the method described herein withoutdeparting from the concept, spirit, and scope of the presently disclosedand/or claimed inventive concept(s).

As utilized in accordance with the present disclosure, the followingterms, unless otherwise indicated, shall be understood to have thefollowing meanings.

The use of the word “a” or “an” when used in conjunction with the term“comprising” may mean “one”, but it is also consistent with the meaningof “one or more”, “at least one”, and “one or more than one”. The use ofthe term “or” is used to mean “and/or” unless explicitly indicated torefer to alternatives only if the alternatives are mutually exclusive,although the disclosure supports a definition that refers to onlyalternatives “and/or”. Throughout this application, the term “about” isused to indicate that a value includes the inherent variation of errorfor the quantifying device, the method being employed to determine thevalue, or the variation that exists among the study subjects. Forexample, but not by way of limitation, when the term “about” isutilized, the designation value may vary by plus or minus twelvepercent, or eleven percent, or ten percent, or nine percent, or eightpercent, or seven percent, or six percent, or five percent, or fourpercent, or three percent, or two percent, or one percent. The use ofthe term “at least one” will be understood to include one as well as anyquantity more than one, including but not limited to, 2, 3, 4, 5, 10,15, 20, 30, 40, 50, 100, etc. The term “at least one” may extend up to100 or 1000 or more, depending on the term to which it is attached; inaddition, the quantities of 100/1000 are not to be considered limiting,as lower or higher limits may also produce satisfactory results. Inaddition, the use of the term “at least one of X, Y, and Z” will beunderstood to include X alone, Y alone, and Z alone, as well as anycombination of X, Y, and Z. The use of ordinal number terminology (i.e.,“first”, “second”, “third”, “fourth”, etc.) is solely for the purpose ofdifferentiating between two or more items and is not meant to imply anysequence or order or importance to one item over another or any order ofaddition, for example.

As used herein, the words “comprising” (and any form of comprising, suchas “comprise” and “comprises”), “having” (and any form of having, suchas “have” and “has”), “including” (and any form of including, such as“includes” and “include”) or “containing” (and any form of containing,such as “contains” and “contain”) are inclusive or open-ended and do notexclude additional, unrecited elements or method steps. The term “orcombinations thereof” as used herein refers to all permutations andcombinations of the listed items preceding the term. For example, “A, B,C, or combinations thereof” is intended to include at least one of: A,B, C, AB, AC, BC, or ABC and, if order is important in a particularcontext, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing withthis example, expressly included are combinations that contain repeatsof one or more item or term, such as BB, AAA, AB, BBC, AAABCCCC, CBBAAA,CABABB, and so forth. The skilled artisan will understand that typicallythere is no limit on the number of items or terms in any combination,unless otherwise apparent from the context.

References to hydrogenation of specific compounds herein are forexemplary purposes only, and the presently disclosed and/or claimedinventive concept(s) can be used with other hydrogenatable compounds.For example, the heterogeneous hydrogenation catalyst may be used toreduce or saturate organic compounds having alkyne, aldehyde, ketone,ester, imine, amide, nitrile, and/or nitro functional groups.

Finally, as used herein any reference to “one embodiment” or “anembodiment” means that a particular element, feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. The appearances of the phrase “in oneembodiment” in various places in the specification are not necessarilyall referring to the same embodiment.

It has been discovered that a solid, heterogeneous hydrogenationcatalyst that can be substantially free of metals (i.e., that is“metal-free”) or not, can be made from a solid material having a surfacewith both Lewis acid and Lewis base sites. While not intending to belimited to the chemical mechanism involved, it is believed that theintroduction of a defect into the solid material “frustrates” a pair ofLewis acid and Lewis base sites resulting in an active hydrogenationcatalyst that can be used in a heterogeneous catalyst system. It isbelieved that the FLPs create a catalytically active surfacefunctionality that permits chemisorption of hydrogen and chemisorptionof a compound containing sp-hybridized carbon and/or sp²-hybridizedcarbon with a structure sufficient to allow chemisorption and subsequentreaction (e.g., hydrogenation) to take place. For example, but withoutlimitation, the compound containing sp-hybridized carbon and/orsp²-hybridized carbon is a compound comprising one or more carbonylgroups.

In one embodiment of the heterogeneous hydrogenation catalyst, it hasbeen found that hexagonal boron nitride (h-BN) can produce aheterogeneous FLP under certain novel and non-obvious modifications.Hexagonal boron nitride (h-BN) can be described as a honeycomb networkof fused borazine rings. The sheets of h-BN are loosely held together byvan der Waals interactions between adjacent boron and nitrogen atoms,which stack in an ABAB fashion.

Pristine sheets of h-BN are exceptionally robust and chemically inert.Hydrogen absorption on the surface of pristine h-BN is endothermic withrespect to dissociation but may be enhanced by introducing vacancies orStone-Wales-type defects into the h-BN sheet. Single layer hexagonalboron nitride sheets can also be thought of as a structurally frustratedLewis pair. Through delamination and the introduction of other defectsin the layers and structure of the materials, the degree of“frustration” within the h-BN molecule can be increased, therebyrendering the h-BN material catalytically active. Defects anddelamination can be introduced through any known physical chemical,and/or electronic process. In one particular embodiment, such defectscan be introduced through the application of mechanical force. When suchdefects are introduced, for example, the lone electron pairs on thenitrogen are free to interact with adjacent layers, but not with theadjacent boron itself. FIG. 1 illustratively depicts an example ofdefective laden h-BN.

As discussed further herein, such h-BN materials having defects givingrise to catalytic activity can be used as hydrogenation catalysts. Theh-BN catalyst material, in particular but without limitation, can beused within any known reactor shape or type and may be used with orwithout a particular refractory material. In one particular embodiment,the h-BN catalyst material is particularly well-suited for use within amechanical reactor (such as a mill) to catalyze hydrogenation. However,the heterogeneous catalysts described and/or claimed herein are notlimited to use in a mechanical/mechanochemical reactor.

Density functional theory (DFT) calculations reveal a 0.56 electrontransfer from B to N for defect-free h-BN sheets. (See Sattler, K. D.Handbook of Nanophysics. Functional Nanomaterials; Taylor & Francis:Boca Raton, 2011, hereby incorporated herein in its entirety).Defect-free h-BN behaves, at least theoretically, as an FLP.Computational studies also show that hydrogen absorption on the surfaceof defect-free h-BN is endothermic with respect to dissociation.Experimental studies have also shown that defect-free h-BN nanotubes canadsorb up to 2.6 mass % of hydrogen. (See Sattler, K.D. Handbook ofNanophysics. Functional Nanomaterials (2011)). As disclosed herein, theintroduction of defects into the structural organization of h-BN resultsin a material having enhanced FLP structural defects on the surface ofthe h-BN material. After the introduction of such defects, the h-BNmaterial exhibits catalytic activity and is capable of being used as ahydrogenation catalyst, for example. Density function theory (DFT)calculations have also previously suggested that carbon dioxide can bestored or activated on pyridine, electron-rich boron nitride, and on theB_(N) defects in BN nanotubes (see Lim et al., Mechanism of HomogeneousReduction of CO2 by Pyridine: Proton Relay in Aqueous Solvent andAromatic Stabilization. Journal of the American Chemical Society, 2012.135(1): p. 142-154; Ertem et al., Functional Role of Pyridiunium duringAqueous Electrochemical Reduction of CO2 on Pt(111). The Journal ofPhysical Chemistry Letters, 2013. 4(5): p. 745-458; and Sun et al.,“Charge-Controlled Switchable CO₂ Capture on Boron NitrideNanomaterials,” Journal of the American Chemical Society, 2013. 135(22):p. 8246-8253, each of which is hereby incorporated by reference in theirentirety), which possess similar electronic and structural motifs asdefect-laden h-BN presently disclosed and/or claimed herein. As such,the defect-laden h-BN catalyst presently disclosed and/or claimed hereinis capable of being a good hydrogenation catalyst for carbon dioxide.

Additionally, hydrogenation over stainless steel (i.e., through the useof a stainless steel catalyst) has recently been observed. (See F. Zhao,Y. Ikushima, M. Arai, “Hydrogenation of 2-butyne-1,4-diol tobutane-1,4-diol in supercritical carbon dioxide.” Green Chem. 5, 656(2003)). In order to eliminate the possibility that the hydrogenationreactions using defect-laden h-BN catalyst material (as disclosedherein) were influenced or catalyzed by the stainless steel reactorcomponents, exemplary catalysis reactions were performed using stainlesssteel components in the absence of defect laden h-BN catalyst material.Limited hydrogenation was observed when stainless steel milling mediawas utilized without the h-BN catalyst material—i.e., the stainlesssteel components contributions to catalytic activity were minor.

As disclosed, the controlled introduction of defects into h-BN producesa reliable and effective heterogeneous hydrogenation catalyst. Inparticular, but not by way of limitation, mechanical processing (e.g.,grinding within a mill or through sonochemical processing) produces adefect-laden h-BN material that has catalytic activity—i.e., aheterogeneous hydrogenation catalyst. Mechanical processing may, in oneembodiment, be required for the initial preparation of the defect ladenh-BN. Grinding h-BN in the presence of hydrogen, for example, inducedhydrogenation in a small batch reactor. However, hydrogenation overdefect laden h-BN can be performed in a fixed bed reactor or otherequipment known to those skilled in the art. For example, solid FLPs,including but not limited to h-BN, can be immobilized in oxygen-freepolymers, such as polysilazanes, to produce materials suitable for fixedbed implementation, as illustrated in FIG. 2.

The presently disclosed and/or claimed heterogeneous hydrogenationcatalyst is not limited to defect laden h-BN. Any solid structurecontaining a non-metallic Lewis acid type moiety and a non-metallicLewis base type moiety can be used provided that the acid and basemoieties are geometrically constrained so that an acid-base adductcannot form. Non-limiting examples of suitable non-metallic Lewis acidtype moieties include elements from Group 13 in a trigonal planarconfiguration; higher halides of Group 15 elements; and electron poorπ-systems such as substitutionally doped graphite or carbon nitride.Non-limiting examples of suitable non-metallic Lewis base type moietiesinclude simple anions such as fluoride and hydride; lone-pair-containingspecies such as Group 15 and Group 16 elements; complex anions such assulfate, selenate, and tellurate; and electron rich π-system Lewis basessuch as substitutionally doped graphite or carbon nitride. Examples ofsolids that would possess these properties include, but are not limitedto, defect laden h-BN, substitutionally doped graphite, substitutionallydoped carbon nitride, and inorganic-organic hybrid materials thatcontain both Lewis acid and Lewis base structures that are constrainedfrom forming an acid-base adduct by the inorganic framework.

Without wishing to be held to any particular hypothesized mode ofaction, hydrogenation over the defect laden h-BN is believed to occurdue to one or more of the following mechanisms: (i) through migration ofprotons over the catalyst surface, as observed in metal-catalyzedhydrogenation, (ii) through chemisorption of the reactant onto aprotonated site, as observed in zeolite-catalyzed hydrogenation, and(iii) through the interaction of bound reactants on separate sheets ofdh-BN. All of these mechanisms require the presence of defects for theinitial chemisorption of hydrogen.

The first mechanism, i.e., hydrogenation through the migration ofprotons over a catalyst, requires mobile defects in the catalyst sinceprotons will be bound to the defect sites. The mobility of protons afterhemolytic bond cleavage may proceed via a mechanism similar to thediffusion of hydrogen on a graphene sheet. See Herrero et al.,“Vibrational properties and diffusion of hydrogen on grapheme,” Phys.Rev. B: Condens. Matter Mater. Phys., 2009, 79(11): p.115429/1-115429/8, hereby incorporated by reference in its entirety.Using a basic bond enthalpy analysis, this mechanism would require asmuch energy as diffusion of H on grapheme which proceeds by breaking andcreation of C—H bonds (C—H, 337.2 kJ/mol, B—H 330 kJ/mol, N—H 339kJ/mol) and be less favored when compared to nickel surfaces (Ni—H 289kJ/mol). Additionally, the V_(N) defects are not mobile but the V_(B)defects are mobile above 840K and the B/N and Stone-Wales defects aremobile under plastic deformation. See Alem et al., “Vacancy growth andmigration dynamics in atomically thin hexagonal boron nitride underelectron beam irradiation.” Physica Status Solidi Rapid Research Letter,2011. 5(i): pp. 295-297; Zobelli et al., “Vacancy migration in hexagonalboron nitride,” Phys. Rev. B: Condens. Matter Mater. Phys., 2007. 75(9):pp. 094104-094104/7; and Zhang et al., “Diffusion and coalescence ofvacancies and interstitials in graphite: A first-principles study.”Diamond & Related Materials, 2010. 19: pp. 124-1244, each of which ishereby incorporated by reference in its entirety. The motion of defectsin BN nanotubes has been observed under applied load and the forcesgenerated during milling are sufficient to exceed the compressive yieldstrength of h-BN (41.3 MPa) when milling intensity is high enough (SeeFIG. 3);

The second mechanism, i.e., hydrogenation through chemisorption of thereactant onto a proton-laden defect, can occur without the applicationof mechanical force. Similar to the diffusion of protons over the dh-BNsurface mechanism, as described above, no change in the catalyticefficiency would be expected when using dh-BN as a hydrogenationcatalyst without the input of mechanical energy;

The third mechanism, i.e., hydrogenation through the interaction ofbound reactants on separate sheets of dh-BN, can occur when a boundolefin on a dh-BN sheet interacts with a bound hydrogen on a separatedh-BN sheet when the two sheets come in close proximity due to mixing.

As mentioned, the presently disclosed and/or claimed inventiveconcept(s) has found that heterogeneous catalysts comprising a solidmaterial having a surface with both Lewis acid and Lewis base sites withat least one defect are capable of reducing carbon dioxide to formicacid. In one embodiment, the heterogeneous catalyst is defect ladenh-BN. Defect laden h-BN is useful as a heterogeneous catalyst because,as illustrated in Table 1, H₂ and CO₂ both have an affinity for thedefects contained therein, specifically the boron rich defects. (SeeChoi, H., Y. C. Park, Y. H. Kim, and Y. S. Lee, Ambient Carbon DioxideCapture by Boron-Rich Boron Nitride Nanotube. Journal of AmericanChemical Society, 2011, 133(7), pp 2084-2087, hereby incorporated in itsentirety). The more negative the binding energy, the stronger thechemisorption of H₂ and CO₂ on the defective sites of the h-BN. Thus,both CO₂ and H₂ are activated in the presence of defect laden h-BN andthe formation of formic acid results.

TABLE 1 Binding Energy (eV) Surface Defects Hydrogen Carbon DioxideBoron Vacancy −5.58 — Nitrogen Vacancy −1.64 — Stone-Wales Distortion0.64 — Nitrogen on Boron Site 0.26 — Boron on Nitrogen Site −2.16 −0.34B/N swap −8.66 —

The presently disclosed and/or claimed invention is also directed to amethod of collecting hydrocarbons indirectly produced using theabove-described heterogeneous catalysts, wherein the catalysts arecoated with at least one or more hydrocarbon during the reduction ofCO₂. In one embodiment, the at least one or more hydrocarbon is removedfrom the catalyst by heating the catalyst to a temperature greater thanabout 100° C., or greater than about 400° C., or greater than about 800°C., and collected by condensation.

As such, the presently disclosed and/or claimed heterogeneous catalystscan be used to chemically reduce CO₂ using relatively low temperaturesand relatively low amounts of energy. The presently disclosed and/orclaimed inventive concept(s) encompasses the use of heterogeneoushydrogenation catalysts for the reduction of CO₂ and the production offormic acid and the indirect formation of hydrocarbon byproducts, whichtogether have a significant commercial and environmental value.

Examples

In one exemplary embodiment, a hydrogenation reactor can be constructedin a manner similar to a pebble mill. Such a pebble mill is capable ofproducing defect laden h-BN by mechanically inducing defects anddelamination of the physical structure of h-BN. The continuous grindingmotion within the mill prevents cluster formation and maximizes thenumber of few-layer sheets of h-BN. In order to verify the presence ofdefect-laden sheets, scanning electron microscopy (SEM) was used tocharacterize the morphology of the defect laden h-BN catalyst. Few-layersheets are observed by SEM and appear to form tubular structures ornano-scrolls as seen in FIG. 1.

The application of mechanical force is, in one embodiment, a preferredmethod for the delamination and formation of defects in the h-BNcatalytic material. Such a delaminating and defect forming force can beapplied during the step of hydrogenation or may be utilized as apretreatment step since the defect sites are stable up to 900° C. (See,P. Wang, S. Orimo, T. Matsushima, H. Fujii, G. Majer, “Hydrogen inmechanically prepared nanostructured h-BN: a critical comparison withthat in nanostructured graphite.” Applied Physics Letters 80, 318(2002)). The application of force during milling requires the efficienttransfer of mechanical force to the h-BN catalyst material.

Synthesis of Defect Laden h-BN

Defect laden h-BN was synthesized by the application of mechanical forceto h-BN under hydrogen for 96 hours in a custom pebble mill, hereafterdescribed in detail, with a gas-tight milling container constructed of304 stainless steel (see FIG. 4). The reaction vessel was shaped as adouble truncated cone to ensure adequate tumbling of the milling media.Conflat flanges with silicone O-rings and Deublin® rotary feedthroughs(Deublin Company, Waukegan, Ill.) with Kalrez® O-rings (E.I. du Pont deNemours and Company, Wilmington, Del.) and Krytox® lubricant (E.I. duPont de Nemours and Company, Wilmington, Del.) were used to maintain gastight conditions during operation. Stainless steel frits (Applied PorousMaterials) were fitted to the entry and exit feedthroughs to eliminatethe accumulation of dust in the sealing surfaces of the feedthroughs.Spherical milling media (440C) was added in the following quantities:twelve 0.75″ balls, sixty-three 0.5″ balls, and eighty-six 0.25″ balls.Temperature was controlled with an Omega® CN3000 process controller(Omega Engineering Inc, Stamford, Conn.) and a K-type thermocouplespring mounted to the inlet flange. Heat was applied by a wound NiChromeheating element embedded in shaped firebrick. Pressure was monitoredwith a NOSHOK® (NOSHOCK, Inc., Berea, Ohio) pressure transducer andcontrolled with a MICROMOD 53MC5000 loop controller (MicroMod Automation& Controls, Rochester, N.Y.). The mill's rotational speed was controlledwith a ⅓ hp variable speed DC motor. An alternative custom pebble millthat may be used to synthesize defect laden h-BN, similar to the onemade out of stainless steel described above, was constructed out ofalumina and used ZrO₂ milling media (FIG. 8).

Hydrogenation

Hydrogenation (also referred to herein as a “reduction reaction”) wasperformed in the custom pebble mill described above for the synthesis ofdefect laden h-BN. The hydrogenation reactions were carried out attemperatures up to 170° C., pressures between 150 and 120 psi, and arotary speed of 60 rpm. Hydrogenations were performed with 2 grams ofthe defect laden h-BN catalyst and a starting pressure of 60 psi of CO₂and 60 psi of hydrogen. Additionally, since formic acid boils at 100.8°C., a cold trap consisting of a Hoke® tee filter (Hoke Inc.,Spartanburg, S.C.) with a stainless steel frit filled with activatedcharcoal was added to the outlet to absorb the formic acid produced. Thetee was kept at 4° C. using a Thermotek® recirculating chiller(ThermTek, Inc., Flower Mound, Tex.). FIG. 5 illustrates the pressureprofile of the reactor over time and depicts the rapid pressure lossmeasured when the reactor reached 170° C. due to the production offormic acid and its subsequent trapping in the activated charcoal coldtrap.

Gas Chromatography with Mass Sensitive Detection

GC-MS analysis was performed on an Agilent 6850 GC (AgilentTechnologies, Santa Clara, Calif.) with an Agilent 19091-433E HP-5MScolumn (5% phenyl methyl siloxane, 30 m×250 μm×0.25 μm nom.) coupledwith a 5975C VL mass selective detector. Activated charcoal samples wereremoved from the filter tee and placed in a 10 mL sealed GC headspacecontainer. The containers were heated to 150° C. and 2 μL of theheadspace gas was sampled. The results of the analysis depicted in FIGS.6 and 7 illustrate the formation of formic acid (FIG. 6) and theincorporation of carbon onto the h-BN catalyst represented as carbonylstructures (FIG. 7).

XPS

The incorporation of carbon onto the catalyst was measured using X-rayphotoelectron spectra (XPS) analysis and TGA analysis, whereby the XPSanalysis confirmed that carbon was in fact bound to the dh-BN surfaceand the TGA analysis indicated a capture of approximately 32.9 masspercent carbon (FIG. 9). The XPS analysis was done using a PhysicalElectronics 5400 photoelectron spectrometer with a magnesium source, theresults of which are presented in FIG. 7. Although the incorporation ofcarbon onto the catalyst turns the catalyst from a shade of white to atan or brown (FIG. 10), the catalytic ability of the defect laden h-BNis not affected.

Collection of Hydrocarbons Produced by the Incorporation of Carbon ontothe Catalyst

The catalyst was regenerated by heating the catalyst in air to 800° C.,returning the color of the catalyst to white. It is anticipated, andwould be understood to one of ordinary skill in the art to be disclosedherein, that other methods exist to remove the carbon-based compositionsfrom the catalyst including, for example but without limitation,oxidizing it to CO. While heating the catalyst, the carbon-basedcompositions incorporated onto the catalyst were removed from thesurface of the catalyst and collected by condensation and thereafterdetermined to comprise carbon in the form of hydrocarbons. As such, thecontrolled introduction of defects into h-BN produces a reliable andeffective CO₂ reduction catalyst that is also capable of producinghydrocarbons from CO₂.

From the above description, it is clear that the inventive concept(s)disclosed herein is well adapted to carry out the objects and to attainthe advantages mentioned herein as well as those inherent in theinventive concept(s) disclosed herein. While exemplary embodiments ofthe inventive concept(s) disclosed herein have been described forpurposes of this disclosure, it will be understood that numerous changesmay be made which will readily suggest themselves to those skilled inthe art and which are accomplished without departing from the scope ofthe inventive concept(s) disclosed herein and defined by the appendedclaims.

1-15. (canceled)
 16. A hydrogenation process comprising: contacting acompound comprising at least one of sp-hybridized carbon andsp²-hybridized carbon with hydrogen gas and a catalyst, wherein thecatalyst comprises a solid material having frustrated Lewis pairs; andcatalytically hydrogenating the compound comprising at least one ofsp-hybridized carbon and sp²-hybridized carbon.
 17. The process of claim16, wherein the compound comprising at least one of sp-hybridized carbonand sp²-hybridized carbon comprises at least one carbonyl group.
 18. Theprocess of claim 17, wherein the compound comprising at least one ofsp-hybridized carbon and sp²-hybridized carbon is carbon dioxide. 19.The process of claim 18, wherein the hydrogenation of carbon dioxideproduces formic acid.
 20. The process of claim 18, wherein thehydrogenation of carbon dioxide coats at least one hydrocarbon onto thecatalyst.
 21. The process of claim 20, further wherein the at least onehydrocarbon is collected by heating the catalyst to a temperaturegreater than about 100° C.
 22. The process of claim 21, wherein thecatalyst is heated to a temperature greater than about 400° C.
 23. Theprocess of claim 21, wherein the catalyst is heated to a temperaturegreater than about 800° C.
 24. The process of claim 16, further whereinthe solid material having frustrated Lewis pairs comprises a solidsurface having at least one Lewis acid site and at least one Lewis basesite, and at least one defect frustrating at least one pair of Lewisacid and Lewis base sites, wherein the at least one frustrated pair ofLewis acid and Lewis base sites is catalytically active.
 25. The processof claim 16, further wherein the solid material having frustrated Lewispairs comprises a solid surface having Lewis acid moieties and Lewisbase moieties spaced a distance apart from one another such thatcatalytic activity is present there-between and the formation of anacid-base adduct therefrom is prevented.
 26. The process of claim 25,wherein the Lewis acid moieties are selected from the group consistingof Group 13 elements in a trigonal planar configuration, halides ofGroup 15 elements, electron poor π-systems, and combinations thereof.27. The process of claim 25, wherein the Lewis base moieties areselected from the group consisting of simple anions,lone-pair-containing species, complex anions, electron rich π-systems,and combinations thereof.
 28. The process of claim 25, wherein the Lewisacid moieties are selected from the group consisting of Group 13elements in a trigonal planar configuration, halides of Group 15elements, electron poor π-systems, and combinations thereof, and theLewis base moieties are selected from the group consisting of simpleanions, lone-pair-containing species, complex anions, electron richπ-systems, and combinations thereof.
 29. The process of claim 25,wherein the Lewis acid moiety is a Group 13 element in a trigonal planarconfiguration and the Lewis base moiety is a lone-pair containingspecies.
 30. The process of claim 25, wherein the hydrogenation catalystat least partially comprises hexagonal boron nitride.
 31. The process ofclaim 16, further wherein the catalyst comprises a sheet ofcatalytically active material having unsatisfied Lewis acid-base pairsalong a surface of the sheet.
 32. The process of claim 16, wherein thecatalyst comprises hexagonal boron nitride having a catalytically activedefect on a surface thereof.
 33. The process of claim 32, wherein thecatalytically active defect is selected from the group consisting ofStone-Wales defects, B/N defects, boron substituted for nitrogen,nitrogen substituted for boron, carbon substituted for nitrogen, carbonsubstituted for boron, boron vacancy, nitrogen vacancy, and combinationsthereof.
 34. The process of claim 32, wherein the catalytically activedefect is boron substituted for nitrogen.